Some features or the entire program may "require a subscription". Which means that the program will refuse to work after a certain period of time, if a <i>certain action</i> will not be made. If such features are implemented we can say that these features are Non-Free, since they deny the zeroth essential freedom: to run the program as you wish and for whatever purpose. If this is implemented in source code and under a Free Software license, though, it's still possible to edit the source code, to remove the time-bomb. And turn the program into a truly Free one. With Non-Free software it's impossible to do so. Regardless, subscriptions are an anti-feature.
